1. Field of the Invention
The present invention relates to medical devices that are delivered into patients by catheters using minimally invasive procedures and methods of compacting such devices.
2. Description of Related Art
Arteriosclerosis affects a significant portion of the population. The progressive nature of the disease can result in severe vessel stenosis (narrowing) and ischemic conditions distal to the stenosis. Although conventional surgical interventions have proven highly effective at treating such conditions, in many cases associated procedural morbidity and mortality has driven the development of alternate xe2x80x9cminimally invasivexe2x80x9d therapies. These therapies are particularly useful when a lesion to be treated is deep within the body, such as in aortic and cardiac vessels or within the skull base (such as, a carotid artery or deep neuro-vasculature). These minimally invasive techniques have enjoyed increasing success and acceptance in the treatment of several vascular diseases including aneurysmal and occlusive disease.
In a typical minimally invasive procedure, upon gaining percutaneous access to the patient""s vascular system, a guidewire is introduced and guided under fluoroscopic visualization to the intended site of therapy. The guidewire then serves as xe2x80x9crailxe2x80x9d onto which other subsequent devices are guided through the vessels to the site. A typical occlusive lesion may require pre-dilation (e.g., PTA or PTCA) and the placement of an endovascular device (such as a stent or stent-graft). This device may then permanently reside within the lumen of the vessel. All components for these procedures are delivered within the vessel (i.e., xe2x80x9cendoluminallyxe2x80x9d) and actuated remotely from outside of the body. Since open surgery is not required, these procedures are considered xe2x80x9cminimally invasive.xe2x80x9d
For the purposes of the following description, endovascular devices may be classified in two general categories: (1) plastically deformable (e.g., balloon expandable); and (2) self-expanding.
Plastically deformable devices are generally deployed by deforming the device at the site of therapy, usually by internal pressure such as inflation of an angioplasty balloon. Devices of this type are generally made of a ductile bio-acceptable material that provides little recoil after dilation. A major advantage of the plastically deformable device is obviating the need for incorporating a restraining device into the delivery system since balloon inflation is all that is needed for proper deployment.
Self-expanding devices, in contrast, are designed to spontaneously deploy in situ once they are released from a constrained profile. They are generally made from some type of elastic, super-elastic, and/or shape memory metal or polymer. Advantages of this type of device are: 1) self-deployment obviates the need for high pressure ballooning at the therapy site; 2) clinical application of self-expanding devices has demonstrated a significant increase in minimum lumen diameter as compared to balloon expandable devices; and 3) super-elastic, pseudo-elastic, and shape memory alloys provide a high degree of compliance and will maintain their expanded profiles despite subsequent mechanical deformation (such as forces that might be encountered in an accident or other pressure applied through a patient""s skin).
Both device categories share a common requirement that they must be introduced to the body from an access site remote to the actual therapy site. As a result, they must be inserted in a first small xe2x80x9cintroductoryxe2x80x9d configuration, guided at this introductory profile through a patient""s vasculature, and deployed through an actuation mechanism to achieve a second xe2x80x9cfunctionalxe2x80x9d configuration.
Many techniques have been developed to configure endovascular devices at a small introductory profile in preparation for insertion to the body. These techniques vary depending upon the category of the individual device.
In the instance of plastically deformable devices, the device may only need to be mechanically crimped onto a balloon prior to insertion to the body. Since this device is made of substantially non-recoiling material, the device, once crimped onto the balloon, will be readily retained on the balloon while being guided to the lesion site.
Although crimping may be done by hand, manual techniques are often unsatisfactory due to non-uniform pressure applied to the crimped device. This can lead to non-uniform device expansion and increased variability in clinical performance. As a result, a number of devices and processes have been developed to reliably and consistently crimp plastically deformable devices onto, or into, a delivery system.
U.S. Pat. No. 5,920,975 to Morales describes a tool that winds a spring-like element around a plastically deformable device while it is mounted upon a delivery balloon. As the spring is tightened, pressure is applied to the device intending to crimp it onto the balloon.
EP Patent Application 630,623 to Williams et al. describes two methods to reduce the cross section of a device. In one embodiment, a plastically deformable device is mounted upon a delivery balloon and placed between reciprocating flat plates. The flat plates act to roll the device while reducing its cross sectional profile. The additions of force and size gauges, as well as inherent consistency of the machine, make this an improvement over the manual crimping technique of rolling the device between fingers.
In another embodiment of Williams et al., a plastically deformable device is mounted on a delivery balloon and then inserted into a chamber. This chamber is lined with a sealed, distensible bladder that, upon inflation, applies a circumferential crushing force to the device. This crushing force is intended to reduce the device profile and securely mount the device on the balloon.
U.S. Pat. No. 6,309,383 to Campbell et al. describes a crimping tool that resembles a hand-held nutcracker or set of pliers. A plastically deformable device is mounted on a delivery balloon and inserted into an orifice in the apparatus. The crimping tool is squeezed to apply pressure to the outside of the device to radially compact the device onto the balloon.
EP Patent Application 903,122 to Morales describes a crimping tool that uses a set of jaws to radially constrict a plastically deformable device onto a delivery balloon. The segmented jaws are hinged on one end to allow them to open and accept a device and its balloon delivery system. Once the device is inside, a collar is slid over the outer surface of the jaws. Pressure applied against the jaws by the collar causes them to close, thereby crushing the device onto the balloon.
In the instance of self-expanding devices, the diametrical size of the device needs to be reduced to an xe2x80x9cintroductoryxe2x80x9d profile and held in place by some constraint. This is generally a more complex procedure than compacting a plastically deformable device since a steady constraint must be applied to the compacted device from its initial compaction to its ultimate deployment. This is typically accomplished using a tool or machine to reduce the device profile, and then the device is transferred in its compacted state to a restraining sheath, catheter, or other constraining means. The constraining means is kept actively engaged up to the time of deployment at the treatment site.
U.S. Pat. No. 6,096,027 to Layne describes an apparatus for crushing and loading a self-expanding device. This device utilizes a bag surrounding the device that is pulled through a tapered die (funnel). As the device moves through the funnel its cross sectional profile is reduced. Upon exiting the die, the bag is removed and the device is captured in a restraining tube or sheath.
U.S. Pat. No. 5,928,258 to Kahn et al. describes an apparatus for crushing and loading a self-expanding device that utilizes a cylindrical cartridge for receiving the device and another implement for transferring the device into a delivery sheath. The device is pulled into the first cartridge, and then a plunger mechanism is used to transfer it to an awaiting delivery sheath or catheter.
U.S. Pat. No. 5,873,906 to Lau et al. describes a method of xe2x80x9cfoldingxe2x80x9d a self-expanding device which entails flattening and rolling the device into a xe2x80x9cjelly rollxe2x80x9d configuration. The device is then restrained in this xe2x80x9cintroductoryxe2x80x9d profile through the use of a fiber based constraining mechanism, and applied to a delivery system. A series of fibers are likewise used to constrain a self-expanding device in U.S. Pat. No. 6,224,627 to Armstrong et al.
Further improvements in compacting self-expanding devices to a minimal introductory profile are disclosed in International Publication No. WO 00/42948 to Vonesh et al., which describes unique fluted funnel designs that allow self-expanding devices to be simultaneously folded and compacted through a funnel to a very low introductory profile.
While all of these prior devices may work well for their intended purposes, it is believed that further significant reductions in introductory profiles may still be possible. Two competing design parameters confront an implantable device designer in maximizing compaction of a device. In addition to having sufficient structural integrity to work for its intended purpose, a compacted implantable device design must balance: (1) the need to limit the amount of material comprising the implantable device so as to have less material to compact; and (2) the need to have a fairly robust implantable device that can withstand the considerable forces encountered in achieving extremely compact dimensions. While a device formed from thinner materials has less material to compact, such a device may not withstand the forces required to reach the smallest possible compacted state. In contrast, a robust implantable device that can be withstand aggressive xe2x80x9cmashingxe2x80x9d to smaller dimensions generally has too much material to achieve a small enough profile. This conflict between minimizing device bulk while maximizing device robustness is most clearly confronted when compacting a self-expanding implantable device through a funnel.
It is believed that the most effective means currently known for compacting a self-expanding device is to pull the device down to a compact size through one or more funnel devices, and particularly through a fluted funnel device. This process is very effective at achieving a small compacted size while imparting minimal damage to the implantable device. Unfortunately, the process of pulling a device through a funnel is limited by the robustness of the implantable device. In order to compact a device in a funnel, the device is attached to tether lines or similar means and then actuated through the funnel. This applies a number of forces to the implantable device, including the force necessary to compact the device as well as the friction forces applied by the funnel and any subsequent restraining means as the device is squeezed through these apparatuses. A thin, lightweight device has the advantage of having minimum material to compact, but such devices tend to pull apart as they are pulled through tight funnels to very small compacted dimensions. More robust devices that can withstand such extreme pulling forces can be provided, but these devices are by necessity bulkier and therefore limited in their ultimate compactability.
It is accordingly a purpose of the present invention to provide an improved method for compacting an implantable device that can achieve a highly compacted introductory profile.
It is a further purpose of the present invention to provide such a method for compacting that does not damage the device in the process of compaction and without the need to have an overly bulky implantable device.
It is still a further purpose of the present invention to provide an implantable device that has a very low delivery profile that is smaller than a profile that can be achieved by pulling the device through a funnel.
These and other purposes of the present invention will become evident from review of the following description.
The present invention provides unique implantable devices that have extremely small introductory profiles, and particularly interventional devices, and methods for achieving such small introductory profiles. The small profiles achieved with the present invention are possible by xe2x80x9cdecouplingxe2x80x9d the forces required to pull an implantable device through a funnel into a retaining device from the forces required to compact the device fully to its introductory profile. For example, forces can be decoupled in the present invention by pulling an implantable device from a fully enlarged profile through a funnel and into a capture tube at an intermediate device profile. The intermediate profile should be one that limits the compaction and friction forces required to compact and capture the device to less than the longitudinal strength of the device. Once placed in the capture tube at the intermediate profile, the capture tube and device are then compressed further to a final delivery profile by swaging the capture tube.
The process of the present invention protects the integrity of the implantable device without requiring the implantable device to be more xe2x80x9crobustxe2x80x9d in order to withstand the cumulative compaction and friction forces of transforming it from its fully enlarged profile to its fully compacted profile.
In one embodiment the present invention comprises a method of reducing the cross sectional dimension of a medical device by providing a medical device having an initial cross sectional profile and a restraining member adapted to receive the medical device at a reduced profile. The profile of the medical device is first reduced, such as using a funnel or similar reduction device, to an intermediate cross sectional profile and then placed at this intermediate profile into the restraining member. Radial compressive force is then applied to the restraining member to further reduce the cross sectional profile of the medical device to a fully compacted profile suitable for delivery into a patient.
The present invention further provides a self-expanding stent with an extremely small introductory profile. The stent is one having a longitudinal tensile strength, an enlarged diameter, and a compacted diameter. Since it is created through the de-coupling process of the present invention, the compacted diameter of said stent is smaller than a diameter that could be obtained using a funnel alone to reduce the stent from its enlarged diameter to its compacted diameter.